It has been known to use centrifugal force generated by a diverging bell shaped mouth of a rotary filament trimmer to pull a filament axially through a hollow drive shaft to extend from a rotary trimmer head as evidenced by U.S. Pat. No. 3,928,911, Dec. 30, 1975 issued to C. B. Pittinger, Jr. for Hollow-Shaft Feed Filament Trimmer. It was recognized here that the filament spins during rotation and the open bellmouth prevents twisting up of the filament clamped in place and extending through the hollow shaft.
However, this patent did not recognize that the rotating filament in the region where it bends outwardly at the open mouth would encounter twisting stresses which heat the filament to a point of catastrophic failure, and thus produces a limited working life. U.S. Pat. No. 4,338,719, Jul. 13, 1982 to R. F. Burkholder for Filament Type Vegetation Trimmer recognized this problem by finding that heat was generated in the filament in the region where it bends outwardly from its axial traversal down the head drive shaft. The solution provided was incorporation of a special curved filament feed tube and accompanying fan mounted in an air tight housing on the cutter head and rotatable therewith for feeding air past the motor to cool it and then into the curved tube passageway for retaining the filament around the outward bend region, thereby to force air through the curved tube to cool the filament at the bend region where it tends to be heated by rotation of the cutter head.
Several disadvantages are presented by this solution to the filament heating problem. The fan structure is complex, expensive and contributes to unwanted added gyro action of the rotary cutting head. Also the cooling air is first heated by the motor so that the air cooling is not effective. Furthermore the rotatable curved tube tends to dynamically unbalance the cutting head.
It is therefore an objective of this invention to provide a simpler, more efficient air cooling system for the filament in a rotary filament vegetation cutter for improving filament life without the foregoing disadvantages.
Other objects, features and advantages of this invention will be found throughout the following description, drawings and claims.